Numerous materials in the chemical, pharmaceutical, foodstuff and other related industries, particularly emulsions, suspensions, high viscosity slurries and liquids, etc . . . contain air or dissolved gases or in the form of small bubbles, which during manufacturing, are necessarily incorporated into the liquid, but must not be contained in the final material. Thus, for example, in the case of photographic emulsions, gas bubbles significantly affect the quality of films or photographic papers manufactured with these emulsions, since the bubbles or small gas bubbles interfere with the volume current in the coating devices, thus causing stripes which render the photographic materials useless.
According to the British patent application 2 008 971, there is provided a method and a device for subjecting liquid, with which a gas is simultaneously transported, to a first reduced-pressure degassing while, in a second stage, this liquid is further degassed by means of a centrifugal rotor, in order to be collected in the form of an annular liquid layer into a pumping element and then to be discharged by means of centrifugal forces against the reduced pressure prevailing in the system.
This device and method exhibits significant drawbacks, because the liquid collects and flows between the centrifugal rotor and the housing walls, as well as between the pump rotors and the walls, this liquid being thus exposed to significant shearing forces. So, the friction generates an uncontrollable emission of heat. The resulting heat involves formation of incrusted aggregates and, when these latter break up, they form small chunks, which in the case of photographic emulsions exhibit more significant drawbacks than small bubbles. If heating is excessive, the quality of the photographic emulsion is adversely affected. Since the liquid is discharged at the periphery of the pump, significant disturbances in the flow occur, which may result again in the formation of small bubbles.
Likewise, according to the German patent 2 147 124, there is provided a device for degassing liquids, wherein the liquid is in vacuo processed in a centrifuge, this liquid spreading out to form a film flowing on the inner wall of the centrifuge rotor, in order to be then distributed into a fraction having few bubbles and a fraction having many bubbles. In order that the bubble-rich fraction may then be separated from the bubble-poor fraction, where the liquid leaves the rotor, directing towards the outside, a blade-shaped overflow element is arranged on one side where the bubble-rich liquid flows, which is fed back again into the rotor in order to be processed, while, on the other side, the relatively bubble-poor liquid is removed at a decreasing peripheral rate. In fact, in comparison with the liquid fed into the rotor before degassing, the liquid discharged there, has a lower bubble content, but sufficient bubbles removal is not achieved.
In fact, all these techniques relate to the degassing of liquid materials. However, for some time, particularly in the field of photographic materials, the techniques for manufacturing the emulsion use cooled gelled mixtures of various components in an aqueous coating composition.
French patent application 2 626 088 discloses a method for providing a photographic composition suitable to form a layer on a support, said composition containing at least a silver halide emulsion as well as the required finishing addenda. The method according to this application consists in:
1) individually preparing, or by group of compatible compounds, various components of the desired layer, comprising at least a silver halide emulsion and solutions or dispersions containing one or more finishing addenda and/or gelatin, and chilling these components in order to solidify each of them,
2) cutting them into chunks,
3) cold-blending, in a solid state, the selected components according to the formulation of the desired layer, or of a portion of this layer,
4) liquefying the resulting solid composition in order to feed it into the coating station.
The components individually prepared in step (1) can be individually cold-stored, either after step (1) or step (2) in the form of chunks, or both after step (1) and step (2).
The mean volume of the chunks must not exceed 2 cm3, in order that the subsequent solid mixture is homogeneous, and preferably, it is less than 0.5 cm3.
In this patent application, the composition is liquefied by means of a device including for example an Archimedean screw and a pump forcing the solid composition into a heat exchanger. However, with such a method, in order to avoid bubbles at the outlet of the melt emulsion, a degassing device of the type described in the EP-A-58 353, must be used. EP-A-58 353 discloses a method and a device wherein the liquid is spread out as a thin film, while applying to it a sudden partial vacuum. The liquid film is subjected to an absolute pressure of 8100 Pa. But, under these pressure conditions, even if bubble formation is reduced, the melt emulsion comprises a large amount of dissolved air and is substantially saturated.
Thus, it is an object of the present invention to improve the degassing of a composition before forwarding it to the coating station.
It is yet another object of the present invention to provide a more simple and more reliable degassing device than those used up to now in the art.
It is yet another object of the present invention to reduce to a minimum the interstitial air amount (i.e. between the solid chunks) and to decrease also the dissolved air amount in the composition itself.
Other objects will appear in the following detailed disclosure.